The invention relates to a technique for forming a fragmented permeable mass of particles in an in situ oil shale retort. More particulary, this invention relates to technique for excavation of a void space in a proposed retort site and explosive expansion of oil shale formation into the void space for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United State has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. Oil shale is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen". Upon heating the kerogen decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale; these generally involve either mining the kerogen-bearing shale and removing it to the surface for processing into shale oil or rubblization and processing of the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597, and 4,043,598 which are incorporated herein by this reference.
These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation, containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the subterranean formation, referred to herein as an "in situ oil shale retort" or "retort." Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale (as described in U.S. Patent No. 3,661,423) includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. Continued introduction of the retort inlet mixture into the fragmented mass, advance the combustion zone through the fragmented mass in the retort.
The combustion gas and that portion of the retort inlet mixture which does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas, which can include hydrocarbons, carbon dioxide generated in the combustion zone and from carbonate decomposition, gaseous products produced in the retorting zone, and any gaseous retort inlet mixture that does not take part in the combustion process is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to form a fragmented mass having a relatively uniform distribution of the void fraction, i.e., a fragmented mass of reasonably uniform permeability, so that oxygen-supplying gas can flow uniformly through the fragmented mass during retorting operations. Techniques used for excavating void spaces in a retorting site and for explosively expanding formation toward the voids can affect the uniformity of particle size or permeability of the fragmented mass. A fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass can avoid bypassing portions of the fragmented mass by retorting gas, which can otherwise occur if there is gas channelling through the fragmented mass owing to non-uniform permeability.
It is desirable that techniques used in excavating and explosively expanding formation within an in situ oil shale retort site provide a means for controlling the void fraction distribution within a fragmented mass being formed so that a reasonably distribution of the void fraction can be provided in the resulting fragmented mass.
An in situ retort formed by excavating a large void in the retort site and then explosively expanding formation above and/or below the void into the void can result in substantially uniform distribution of the void fraction throughout the rubblized mass of formation particles in the retort. Such a technique has certain hazards for miners excavating such a void because of the large unsupported expanses under which the miners would have to work. This is true even though oil shale, especially that found in the Western United States is generally considered to be quite competent and can remain open even for substantial periods with relatively large spans. It is, therefore, desirable to provide a technique for forming an in situ oil shale retort wherein the excavation of an underlying void can be performed from a safe location.